7.18 is asking you to first find $a_6$ and $a_4$, and then subtract $a_6-a_4$.

The sequence of numbers in problem 7.21 is going to be given by the number of diagonals in a regular $3$-gon (equilateral triangle), a regular $4-$ gon (square), a regular $5$-gon (regular pentagon) and a regular $6$-gon (regular hexagon).